This invention relates generally to computed tomography imaging systems, and more particularly to post-patient collimators used in such systems and methods for making such collimators.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
In a multislice imaging system, the detector comprises a plurality of parallel detector rows, wherein each row comprises a plurality of individual detector elements. A multislice detector is capable of providing a plurality of images representative of a volume of an object. Each image of the plurality of images corresponds to a separate xe2x80x9cslicexe2x80x9d of the volume. The thickness or aperture of the slice is dependent upon the thickness of the detector rows. It is also known to selectively combine data from a plurality of adjacent detector rows (i.e., a xe2x80x9cmacro rowxe2x80x9d) to obtain images representative of slices of different selected thicknesses.
It is known to provide multislice CT detectors with a post-patient collimator. These collimators include many precisely aligned plates and wires to collimate x-rays impinging on and to attenuate x-rays impinging between individual scintillating detector elements. In one known system, alignment of the collimator plates and attachment of the wires is accomplished with slots and notches in various components for alignment, and adhesives for bonding. The manufacturing steps presently required for precision alignment of the collimator plates and wires add considerably to manufacturing costs. For example, to manufacture one known collimator, upper and lower combs with precision slots, slot spacings, and slot alignments are required for insertion of collimator plates. Welding has not been practical in known post-patient collimators because of induced distortions in collimator plates resulting from the welding process itself.
It would therefore be desirable to provide precision-aligned post-patient collimators for CT imaging systems and methods for manufacturing them that are more efficient and less expensive than those that require precision combs.
There is thus provided, in one embodiment of the present invention, a method for constructing a post-patient collimator for a computed tomographic (CT) imaging system, the method including steps of: edge welding collimator plates to a top rail using at least one directed energy beam welder; and edge welding the collimator plates to a bottom rail, using the at least one directed energy beam welder.
The above described embodiment provides an efficient and less expensive method for manufacturing a post-patient collimator for a CT imaging system than embodiments requiring use of precision combs for accurately positioning the plates.